WELCOME to the Forum, AliRoll!
Petzl Tikka, Pixa or similar should be ideal. It should be a good quality robust waterproof/water resistant light that can be unquestionably relied upon to function in the cave; it should not be a "second best" piece of kit or something from a boot sale, hedgerow etc..
A spare light is arguably best worn around your neck in case you need it in a hurry (i.e. main light failure half way through an exposed traverse), hence one with an elasticated strap is more suitable (see above). Furthermore, the most likely deployment of your backup light is to "save the day" when one of your fellow caving lame duck team members turns up late with half their kit missing and with a totally flat main light, or even missing altogether, so a Tikka/Pixa is an effortless solution.
No longer a fan of helmet mounted lamps (see above) because in addition a) they make you look like a wanna-be cave diver, especially if you have a "matching pair" like a Dalek, b) if your chin strap fails on a pitch you'll lose all your lights at the same time when the helmet falls off, c) in narrow stuff it compromises your ability to turn your head, d) the spare gets scuffed up and looks worn out years before it's even had much of a useful working life.
Prior to EVERY trip check your backup has fully charged batteries and is working.